Jökull


Jökull - 01.01.2012, Side 154

Jökull - 01.01.2012, Side 154
S. Brynjólfsson et al. to be triggered from within or under a glacier rather than to be externally forced (Kamb, 1987; Björnsson, 1998; Kjær et al., 2006; Benn and Evans, 2010), al- though a recent study by Striberger et al. (2011) sug- gests that surge periodicities of the Eyjabakkajökull glacier, eastern Iceland may have been forced by cli- matically driven mass balance changes. Thórarinsson (1964, 1969) compiled historical records and reviewed the state of knowledge on surging glaciers in Iceland with emphasis on outlet glaciers draining the Vatnajökull ice cap. He con- cluded that surging glaciers in Iceland were character- ized by a smooth ablation zones and shallow spoon- shaped basins that widened towards the glacier ter- minus. Later, Björnsson et al. (2003) extended Thór- arinsson’s work and gave an overview of surging glaciers in Iceland and their activity. They suggested that surges affect about 70% of glaciers in Iceland, and that steep-sloped glaciers typically do not surge. Búrfellsjökull and Teigarjökull, located in the moun- tainous Tröllaskagi peninsula, northern Iceland, are surge-type cirque glaciers with mean slopes about 11– 14◦, which make them the steepest surge-type glaciers in Iceland. Judging from the last surge of Búrfells- jökull which lasted for four years, 2001–2004, the surging phase seems to last longer in north Iceland compared to the larger outlet glaciers in south Iceland where surges usually last from a few months to one or two years. Palaeoclimate and glacier fluctuations over time have been reconstructed from marginal moraines and glacial deposits in the Tröllaskagi peninsula, assum- ing that the Tröllaskagi glaciers generally are in equi- librium with climate (Kugelmann, 1991; Caseldine and Stötter, 1993; Stötter et al., 1999). Only three glaciers among more than 150 glaciers in the Trölla- skagi peninsula have been known for surge activity, but a preliminary study by Brynjólfsson (2009) sug- gested that there might be more surge type glaciers in the area. In this study we explore the geomorphology and sedimentology of the forefield of two surge-type cirque glaciers and one non-surging cirque glacier at Tröllaskagi peninsula. Different geomorphological and sedimentological signatures of the surge-type and the non-surging glacier were used to identify specific characteristics of the surge type glacier forefield. Here we describe the geomorphology of the surge- type Búrfellsjökull and Teigarjökull cirque glaciers in Tröllaskagi, explore their dynamics and reconstruct the recent surge history. Our study of those two glaciers results in a new landsystems model for surge- type cirque glaciers in alpine environments. SETTING AND KNOWN SURGE EVENTS The Tröllaskagi peninsula is a mountainous area in central northern Iceland, extending from the central highlands, north out into the North Atlantic Ocean (Figure 1). The glacially sculptured landscape, mostly heavily eroded and dissected Tertiary plateau basalts, ranges in altitude from sea level to more than 1500 m a.s.l. (Björnsson, 1979; Pétursson and Jónsson, 2006). There are more than 150 small glaciers in the moun- tain range (Figure 1), located in cirques and valley bottoms between 700 and 1400 m a.s.l. (Sigurðsson and Williams, 2008). Approximately 40 km2 of the area is covered by glaciers. In most cases, glaciers face north and are sheltered by steep mountain slopes which reduce the incoming solar radiation. Most of the glaciers have surface areas of 0.5–2 km2 and only a few of the glaciers are larger than 2 km2 (Björns- son, 1979; Björnsson, 1991; Björnsson and Pálsson, 2008). The precipitation in the high mountains of Tröllaskagi is estimated to be 2000–2500 mm/year of which the majority falls as snow on the glaciers (Ólafsson and Ólafsson, 2004; Lippert et al., 2006; Brynjólfsson and Ólafsson, 2008). In addition, the glaciers receive accumulation from avalanches and snowdrift (Björnsson 1991, Björnsson and Pálsson, 2008). Studies on mountain permafrost conditions in Iceland and local climate of the Tröllaskagi peninsula show that sporadic permafrost occurs above 800–900 m a.s.l. which is consistent with the occurrence of rock glaciers at high elevation in Tröllaskagi (Farbrot et al., 2007a, 2007b). The study area consists of two small glacially eroded hanging valleys, the Búrfellsdalur and Teigar- dalur that are southern tributaries to the main valley 152 JÖKULL No. 62, 2012
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